The field of the invention is medical imaging and particularly, methods for reconstructing images from acquired image data.
In a computed tomography system, an x-ray source projects a cone-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the “image plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce what is called the “transmission profile,” or “attenuation profile,” or “projection.”
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. The transmission profile from the detector array at a given angle is referred to as a “view,” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This image reconstruction process converts the attenuation measurements acquired during a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
Radiation therapy is a treatment technique that delivers radiation to a defined target volume in a subject. The radiation is delivered in such a manner that the surrounding healthy tissue does not receive radiation doses in excess of allowed tolerances. In order to achieve this control of the imparted dose to the subject, highly accurate radiation delivery techniques are required. Many factors provide difficulties in obtaining the desired level of accuracy, including differences between the planned and delivered dose distributions and uncertainty in subject position with respect to the treatment system.
Intensity modulation radiation therapy (“IMRT”) is a radiation therapy technique that utilizes computer planning software to produce a three-dimensional radiation dose map, specific to a target tumor's shape, location, and motion characteristics. Because of the high level of precision required for IMRT methods, detailed data must be gathered about tumor locations and their motion characteristics. In doing so, the radiation dose imparted to healthy tissue can be reduced while the dose imparted to the affected region, such as a tumor, can be increased. In order to achieve this, accurate geometric precision is required during the treatment planning stage.
Image-guided radiation therapy (“IGRT”) employs medical imaging, such as computed tomography (CT), concurrently with the delivery of radiation to a subject undergoing treatment. In general, IGRT is employed to accurately direct radiation therapy using positional information from the medical images to supplement a prescribed radiation delivery plan. The advantage of using IGRT is twofold. First, it provides a means for improved accuracy of the radiation field placement. Second, it provides a method for reducing the dose imparted to healthy tissue during treatment. Moreover, the improved accuracy in the delivery of the radiation field allows for dose escalation in the tumor, while mitigating dose levels in the surrounding healthy tissue. Exemplary IGRT methods include the use of two- and three-dimensional imaging techniques. Additionally, so-called “four-dimensional” imaging techniques, in which a time series of three-dimensional images are obtained, may be employed during IGRT. In three-dimensional IGRT, imaging is performed using cone-beam computed tomography (“CBCT”), while in two-dimensional IGRT, planar digital radiographs, fluoroscopy image frames, or megavoltage (MV) images are matched with digital reconstructed radiographs (DRRs) from the radiation delivery plan. Recently developed CBCT based IGRT systems include systems where a CBCT is integrated with medical linear accelerator. With improvements in flat-panel technology, CBCT has been able to provide volumetric imaging, and allows for radiographic or fluoroscopic monitoring throughout the treatment process.